The present invention relates to rapid thermal processing (RTP) systems of the type used to fabricate integrated circuits and other electronic devices carried on substrates. More specifically, the present invention relates to an optical heating device for use in a rapid thermal processing (RTP) system.
Integrated circuits and many other electronic devices are built in and on semiconductor wafers and other round substrates. Many early fabrication systems processed batches of small diameter wafers. The wafer size has increased from 1.25 inches in diameter through today's conventional wafer of 200 mm diameter. Fabrication facilities are being designed around the newest wafer standard, 300 mm, with 450 mm wafers expected to be in production by the year 2007. As wafer sizes have increased, film thickness and feature sizes have decreased. In the most extreme case, microprocessors are now being fabricated in production with insulating layers that are approximately 10 atoms thick. Both the larger wafer size and the new process technology have put extreme uniformity and reproducibility requirements on the individual process tools. As a result, with only a few exceptions, process tools are shifting to single wafer reactors.
Since most process steps involve a chemical reaction and/or a film growth, some technique for controlling the temperature of the wafer, typically by a type of wafer heating, is almost always used in these process tools. Individual wafer heating is typically done optically in a process generically known as rapid thermal processing (RTP), or through resistively heated coils or pedestals which are placed in contact with a susceptor on which the wafer is mounted. Resistive heating arrangements suffer from a number of drawbacks. The wafer must beheld in contact with the susceptor. This produces unwanted particulate contamination. The newest technologies are sensitive to particles on both sides of the wafer, and so will find susceptor heating highly unacceptable. When used in conjunction with film deposition processes such as chemical vapor deposition and sputtering, the films accumulate on the heated susceptor, eventually flaking off when the film becomes sufficiently thick. Since the susceptor is hot, it can also outgas leading to memory effects and other unwanted contamination. Thermal uniformity in a resistively heated system often depends on the quality of the contact to the wafer. Variations lead to nonreproducible results. Finally the thermal mass of the susceptor necessitates slow heating and cooling, leading to long process times.
RTP systems have gone through a series of evolutionary improvements. An optical source is isolated from the chamber through a fused silica window which is sealed through the use of elastomer o-rings. This allows the operation of the light source in one atmosphere of air. Early RTP systems were built in two configurations. In the first a single high intensity source such as a high power xenon arc lamp, was used with a shaped reflector to produce an approximately uniform radiant field at the surface of the wafer. The light source while extremely intense and efficient, was difficult to operation and would age nonuniformly. As a result the thermal uniformity at the surface of the wafer would shift over time. The second configuration, which was popular for many years, involved the use of a reflecting cavity to distribute the radiation to the wafer. The wafer was loaded in a fused silica flow tube. Linear filament lamps were arranged above and below the flow tube to act as a heat source. The flow tube and lamps were enclosed in a reflecting box, typically gold plated aluminum or stainless steel. The surface of the cavity was often roughened to randomize the direction of the reflected light rays. Once again, the intention was to produce a uniform radiant field at the surface of the wafer.
Both types of systems suffer from a common deficiency. When a nominally uniform radiant intensity is applied to the top of the wafer, the edges of the wafer were found to be significantly cooler than the center. The reasons for this radial temperature radiant include a variation of the optical view factor (and therefore radiant intensity) across the surface of the wafer, the lack of optical energy directed toward the sides of the wafer, and natural convection in the chamber which tends to overcool the wafer edges. In the later versions of the single lamp system, these effects were sometimes compensated by selectively frosting the fused silica window to scatter some of the light that would otherwise reach the center of the wafer. The temperature variation, however, was not constant. It depended on the process temperature, the optical properties of the wafer, and the gas conditions inside the reactor. Thus the single source system could only optimize for one set of conditions. The reflecting cavity approach to RTP compensated for edge effects by increasing the power density to the lamps near the perimeter of the wafer. Typically the upper and lower lamp banks were arranged perpendicular to each other and parallel to the lane of the wafer. Thus, the extra radiation was distributed in a square about the round wafer, with two side of the square above the wafer and two sides below the wafer. By varying the power distribution between the majority of the lamp array and these edge lamps, one could compensate for edge effects over a range of process conditions.
While the reflecting cavity was widely used for 100 mm to 150 mm wafers, the technique did not have sufficient uniformity to be used with 200 mm systems. For this type of system a more refined zone heating system would be required. The goals of this zone heating are as follows: 1) to collect as much light from the lamps as possible; 2) to be able to distribute that light in circular zones that can be independently controlled to be able to obtain a uniform temperature across the surface of the wafer; and 3) to be as insensitive as possible to lamp to lamp variations in optical conversion efficiency. The latter point is quite important as lamp outputs can vary by as much as 20% as received. After operation, the lamp aging further increases this disparity.
One system that falls into this category is described in U.S. Pat. No. 5,446,825 by Moslehi et al. in which much shorter, single-sided filament lamps penetrate the reflector. Flat sided rings are machined into the reflector to direct some of the light corresponding rings on the surface of the wafer. By arranging the wafers into these circular rings, the light from several lamps is effectively averaged, and so the system is less sensitive to lamp to lamp variation. Collection efficiency was only moderate however. A 100 mm system typically requires .about.50 kW of energy, well in excess of that required for a reflecting cavity design. A 200 mm system requires .about.200 kW.
A second type of zone heated RTP system, now in widespread production for use at 200 mm wafers and beyond, uses an array of lamps, each of which has an individual reflector to collect the light and direct it to the surface of the wafer. This arrangement is sometimes called a spotlight system. Groups of lamps, in roughly the shape of concentric rings, are run in parallel from 3 to 5 power supplies to be able to control the radiant power distribution across the wafer. This type of system has a high collection efficiency, and allows the use of the zone heating technique, but requires careful lamp matching and frequent lamp changes to ensure the necessary uniformity. A typical system requires 137 kW lamps to hear a 200 mm wafer. 200 mm systems are expected to require close to 300 lamps.
While spotlight systems have achieved a great deal of success systems used to primarily to thermally anneal and/or oxidize the wafer, the technique is generally prohibitively expensive and too complicated to operate to be used as a heat source for other processes. Typical spotlight annealing systems are very expensive and require frequent lamp change outs using vendor qualified (i.e., presorted) lamps. As a result, very few equipment vendors have chosen to use the spotlight system for heating wafers in non-annealing processes. Finally both of the zone heating systems described above were designed to heat wafers to temperatures in excess of 1100.degree. C. to anneal implanted impurities in the semiconductor wafer. Most non-annealing processes are run at temperatures well below 700.degree. C. Power requirements for these lower temperature processes are at least four times lower than conventional implant annealing. Operating the heater in this low power regime further degrades the lamp as the operating power is too low to activate the halogen cycle of lamps.
Thus there is a need for a much lower cost version of a multizone lamp module that can heat a semiconducting wafer using cylindrical symmetry. This could be used for low and moderate temperature processes (temperatures less than or equal to 800.degree. C.), including those operations such as plasma assisted processes, where wafer heating plays a supplementary role. The heating arrangement should have a high collection efficiency to minimize operating costs and should allow the overlapping output of multiple lamps in a zone to avoid the localized hot spots that spotlight systems are prone to.